1. Field of the Invention
Relates to method for removing aluminum from crystalline aluminosilicate zeolites while retaining high degree of crystallinity to yield products of enhanced thermal stability and improved catalytic characteristics.
2. Description of the Prior Art
It has heretofore been known to effect removal of aluminum from crystalline aluminosilicate zeolites. Thus, utilizing known techniques such as described in U.S. Pat. No. 3,442,795, it has been possible to remove up to about one-half of the tetrahedrally coordinated aluminum from crystalline aluminosilicate zeolites having an initial silica/alumina ratio of at least about 2:1 but less than about 6:1.
One general method which has been used to accomplish such removal of aluminum involves the reaction of the sodium or ammonium form of the zeolite with ethylenediamine-tetraacetic acid (H.sub.4 EDTA) in accordance with the following equation:
x H.sub.4 EDTA + NaAlO.sub. 2 (SiO.sub.2).sub.y .fwdarw. x NaAl EDTA .multidot. H.sub.2 O + (NaAlO.sub. 2)1-x (SiO.sub.2).sub.y + x H.sub.2 O
where x &lt; 1 and Y &gt; 2.5
This reaction is conveniently conducted in a Soxhlet extractor. The zeolite, slurried in water, is contained in the flask maintained at an elevated temperature and the H.sub.4 EDTA is contained in the Soxhlet thimble. In such manner, a dilute solution of the acid is added generally over a 16 to 24 hour period to the zeolite. Attempts to remove more than 50 to 60 percent of th aluminum via this method have resulted in severe losses in crystallinity and thermal stability.